1. Technical Field
The present document relates to a magnetic resonance imaging apparatus.
2. Related Art
Magnetic resonance imaging apparatuses are apparatuses that are configured so as to obtain an image of the inside of an examined subject (hereinafter, “subject”) by using a magnetic resonance phenomenon. A magnetic resonance imaging apparatus includes various types of units as follows, for example, that are necessary in an image taking process: a static magnetic field magnet that generates a static magnetic field in an image taking area; a gradient coil that applies the gradient magnetic field to a subject placed in the static magnetic field; and a high frequency coil that receives a magnetic resonance signal from the subject to which the gradient magnetic field has been applied. Among these various types of units, there are some units that generate heat during scanning processes.
For example, the gradient coil significantly generates heat during scanning processes because a pulsed electric current is repeatedly supplied thereto according to a pulse sequence. The gradient coil is provided with iron shims used for correcting nonuniformity of the static magnetic field within the image taking area. When the temperature of the gradient coil changes, magnetic susceptibility of the iron shims gets affected and also changes.
When the magnetic susceptibility of the iron shims changes, a change occurs in uniformity of the static magnetic field within the image taking area, and in particular, the center frequency significantly changes. Such changes in the center frequency can be a cause of hindrance for fat suppressions or a cause of artifacts in the images. Accordingly, to obtain images having a stable image quality, it is important to inhibit the changes in the temperature of the gradient coil. In particular, because more powerful gradient magnetic fields are being used in recent years, how to control the heat generated by the gradient coil is becoming an even more important issue.
Generally speaking, to address this issue, Magnetic Resonance Imaging (MRI) apparatuses are each provided with a cooling device for cooling the heat generating sources such as the gradient coil. The cooling device includes, for example, a heat exchanger and a circulating pump and is configured so as to cool the heat generating sources by causing a cooling medium (e.g., water) to circulate through a cooling pipe provided in the surroundings of the heat generating sources (see, for example, Japanese Patent Application Laid-open No. 2006-311957).
The conventional technique described above, however, has a problem where, as explained below, it is not possible to inhibit the changes in the temperature of the heat generating sources according to the magnitude of the heat generated during the scanning processes.
For example, according to the conventional technique described above, in the case where the heat generated by a heat generating source has increased during a scanning process, it is necessary to increase the flow rate of the cooling medium for the purpose of lowering the temperature of the heat generating source. However, as for the gradient coil, for example, because the diameter size of the cooling pipe is limited by structural limitations, it is not possible to increase the flow rate of the cooling medium beyond a predetermined amount. As a result, it is not possible to inhibit the rise in the temperature of the gradient coil beyond the limit of the maximum flow rate.
Alternatively, for example, in the case where the cooling medium is configured so as to have a lower temperature, although it is possible to keep the temperature of the gradient coil low before the scanning process is started, the temperature of the gradient coil rises after the scanning process is started if the temperature of the cooling medium remains the same. As a result, even if the cooling medium is configured so as to have the lower temperature, it is not possible to inhibit the changes in the temperature of the gradient coil during the scanning process.